This invention generally relates to processes for the removal of undesirable material from a substrate surface, and more specifically to a process for the removal of organic materials such as photoresist and contaminants in the photoresist and on the surface of the substrate, which substrate is used for the production of semiconductor devices. The process is especially effective for quickly stripping such materials without significant radiation damage to the substrate. It is thus particularly applicable to automated wafer processing where speed is critical, but effective removal of undesirable materials may not be sacrificed.
Photoresist films are used to mask underlying layers during an etch process. During the etch process, contamination is introduced from several sources, such as from the photoresist itself, from the etched material, and from the etch process by-products. After the etch process is complete, the photoresist is removed.
Photoresist removal was once accomplished by wet chemical means using various acids and solvents, however, dry processing has become the preferred means for reasons of process cleanliness, operator safety, and hazardous waste disposal. Numerous means for dry removal of organics are known.
It may be noted that the terms organic polymer removal, photoresist removal, photoresist etching, photoresist stripping, ashing, plasma cleaning, plasma treatment, dry processing, and gas plasma vapor etching have been used more or less interchangeably in the prior art.
Early dry processes (U.S. Pat. Nos. 3,615,956, 3,705,055, 3,837,856) taught the use of radio frequency (RF) excitation of oxygen gas at pressures of 0.5 to 10.0 torr to produce oxygen radicals and ions which chemically attack the organics to form volatile products such as carbon monoxide, carbon dioxide, water vapor, and low molecular weight organics
Subsequently, improved processes (U.S. Pat. Nos. 3,867,216, 3,879,597, 3,951,843, 4,304,983, 4,443,409, 4,474,621) were proposed, variously adding halocarbons to the oxygen reactant, shielding the substrates from direct contact with the ions produced in the plasma, and directing the reactant gas flow in a jet stream at the substrates. In one particular case (U.S. Pat. No. 3,951,843) it is claimed that tin contamination present in photoresist can be removed as volatile tin tetrachloride (SnCl.sub.4). The rate of removal of organic materials in these processes is relatively slow and non-uniform, but processing substrates in batches of up to 50 pieces at one time helped to make the processes acceptable in a production environment. Some of these systems are referred to as "barrel" etch systems because the wafers are processed within a barrel-like container.
As etching processes became more aggressive, post-etch photoresist stripping became more difficult because highly cross-linked polymers were formed in the photoresist during ion bombardment-type dielectric and metal etch processes. The removal of these highly cross-linked polymers has been accomplished (U.S. Pat. Nos. 4,357,203 and 4,370,195) by sputter-etch techniques and by reduction in a hydrogen plasma. These techniques, however, require treatment times of up to one hour to remove very small amounts of material, and are, therefore, not suited to the mass production of semiconductor or other devices.
More recently, there has been a move away from batch type barrel etch systems toward single wafer systems because the latter are more readily adapted to full automation of the loading and unloading of substrates. Single substrate processing requires much higher process rates in order to compete on an economic basis with batch processing, but it was feared that higher rates of reaction would also require higher RF power levels to accomplish the same degree of stripping in a shorter time. Higher RF power would mean a more aggressive plasma environment with more ion bombardment and stronger UV exposure of the substrates, which are not desirable because of the surface damage and charge-trapping that they can cause.
In order to generate sufficient reactive species without generating radiation damage, RF excitation of the process gas mixture has been replaced by UV excitation (U.S. Pat. No. 4,540,466) or microwave excitation (U.S. Pat. Nos. 4,673,456, 4,705,595, 4,718,976, 4,736,087, 4,804,431 4,836,902) or a combination of UV and microwave excitation (U.S. Pat. Nos. 4,687,544, 4,689,112, 4,699,689, 4,718,974). In each case, one of the stated objectives was to isolate the substrates being processed from direct exposure to ions formed in the high intensity plasma. These processes are sometimes referred to as "remotely excited" or "downstream" processes. By varying process temperatures, gas flows, pressures, and excitation power, stripping rates between 0.5 and 3.0 microns per minute are said to be achieved.
In addition to their claim of preventing ion contact with the substrates, the remotely excited processes typically teach that UV exposure of the substrates should be limited or prevented altogether. On the other hand, processes have also been proposed wherein it is stated that UV exposure is desirable (U.S. Pat. Nos. 4,417,948, 4,687,544, 4,689,112, 4,699,689) as a process initiating or rate enhancing feature.
Oxygen and ozone source processes have also been proposed which use relatively high (atmospheric level) process pressures to achieve high removal rates (1.0 to 3.0 microns per minute) of carboncontaining films (U.S. Pat. Nos. 4,555,303 and 4,812,201)
In all of this prior art, however, it is necessary to follow the strip process with a substrate rinse in acid or solvent to effect the removal of nonvolatile residues, particularly chemically resistant polymers and metal and alkali earth metal contaminants which have been left on the surface or driven or diffused into the substrate surface as a result of exposure to the strip process.